The present invention pertains generally to systems and methods for the destruction of high-level radioactive waste. More particularly, the present invention pertains to methods for converting the spent fuel from a nuclear reactor into a form which is suitable for long term storage at a repository. The present invention is particularly, but not exclusively, useful for transmuting Plutonium239 and other transuranics found in spent nuclear fuel into more stable, less radiotoxic materials.
It is well known that spent nuclear fuel is highly radiotoxic and poses several challenging threats to mankind, including nuclear proliferation, radiation exposure and environmental contamination. To date, approximately 90,000 spent fuel assemblies containing about 25,000 tons of spent radioactive fuel are stored in the United States. Furthermore, with additional spent fuel assemblies being generated each year, it is estimated there will be about 70,000 tons of spent fuel waste by the year 2015. At the rate waste is produced by the existing nuclear reactors in the United States, new repository capacity would be needed every 20-30 years equal to the statutory capacity of the yet-to-open Geological Repository at Yucca Mountain. Currently, about 95% of this radiotoxic material is temporarily stored at the point of generation (i.e. at the power plant) in water pools, with a small amount being stored in dry storage (casks).
A typical spent fuel assembly removed from a commercial nuclear power plant, such as a Light Water Reactor, contains four major constituents: Uranium (about 95%), fissile transuranics including Plutonium239 (0.9%), non-fissile transuranics including certain isotopes of Americium, Plutonium, Curium and Neptunium (0.1%), and fission products (balance). After a relatively short time, the Uranium and a portion of the fission products are generally no more radiotoxic than natural Uranium ore. Consequently, these components of the spent fuel do not require transmutation or special disposal. The remaining fission products can be used as a burnable poison in a commercial reactor followed by disposal at a repository.
The fissile and non-fissile transuranics, however, require special isolation from the environment or transmutation to non-fissile, shorter lived forms. Destroying at least 95% of these transuranics followed by disposal in advanced containers (i.e. containers better than simple steel containers) represents a much better solution than merely stockpiling the waste in the form of fuel rods. In one transmutation scheme, the transuranics are transmuted in a reactor, followed by a separation step to concentrate the remaining transuranics, followed by further transmutation. Unfortunately, this cycle must be repeated 10-20 times to achieve a desirable destruction level of 95%, and consequently, is very time consuming and expensive.
In another transmutation scheme, fast neutrons are used to transmute the non-fissile transuranics. For example, fast neutrons generated by bombarding a spallation target with protons are used. Although these fast spectrum systems generate a large number of neutrons, many of the neutrons are wasted, especially in subcritical systems. Further, these fast neutrons can cause serious damage to fuel and structures, limiting the useful life of the transmutation devices.
In light of the above, it is an object of the present invention to provide devices suitable for transmuting fissile and non-fissile transuranics to achieve relatively high destruction levels without requiring multiple reprocessing steps. It is another object of the present invention to provide systems and methods for efficiently transmuting fissile and non-fissile transuranics with thermal neutrons. It is yet another object of the present invention to provide systems and methods for efficiently transmuting fissile and non-fissile transuranics which use neutrons released during the fission of fissile transuranics to transmute the non-fissile transuranics.
In accordance with the present invention, a system and method for transmuting spent fuel (i.e. radioactive waste) from a nuclear reactor, such as a Light Water Reactor, includes the step of separating the waste into components. For the present invention, a conventional UREX process can be used to separate the spent fuel into components that include a Uranium component, a fission products component, a driver fuel component and a transmutation fuel component. After the separation, the driver fuel and transmutation fuel components are placed in a reactor with a thermal neutron spectrum for transmutation into less hazardous materials. On the other hand, the Uranium component is relatively non-radioactive and can be disposed of without transmutation. Also, the fission products may be transmuted into short-lived, non-toxic forms in commercial thermal reactors.
The driver fuel, which includes fissile materials such as Plutonium239, is used to initiate a critical, self-sustaining, thermal-neutron fission reaction in the first reactor. The transmutation fuel, which includes non-fissile materials, such as certain isotopes of Americium, Plutonium, Neptunium and Curium, is transmuted by the neutrons released during fission of the driver fuel. The transmutation fuel also provides stable reactivity feedback and makes an important contribution to ensure that the reactor is passively safe. The system is designed to promote fission of the driver fuel and reduce excessive neutron capture by the driver fuel. More specifically, the system is designed to minimize exposure of the driver fuel to thermal neutrons within an energy band wherein the driver fuel has a relatively high neutron capture cross-section and a relatively low fission cross-section. In one implementation, the driver fuel is formed into spherical particles having a relatively large diameter (e.g. approximately 300 xcexcm) to minimize neutron capture by the so called self-shielding effect.
The transmutation fuel is formed into relatively small, substantially spherical particles having a diameter of approximately 150 xcexcm in diameter (or diluted 250 xcexcm particles) to maximize exposure of the small amount of the transmutation fuel to epithermal neutrons (i.e. thermal neutrons at the high energy end of the thermal neutron energy spectrum). These neutrons interact with the transmutation fuel atoms in the so-called resonance epithermal region and destroy them in a capture-followed-by-fission sequence. Additionally, the particles are placed in graphite blocks which moderate neutrons from the fission reaction. A relatively high ratio of graphite mass to driver fuel mass is used in the first reactor to slow down neutrons to the desired energy levels that promote fission over capture in the driver fuel.
The driver fuel and transmutation fuel remain in the first reactor for approximately three years, with one third of the reacted driver fuel and transmutation fuel removed each year and replaced with fresh fuel. Upon removal from the first reactor, the reacted driver fuel consists of approximately one-third transuranics and two-thirds fission products. The transuranics in the reacted driver fuel are then separated from the fission products using a baking process to heat up and evaporate volatile elements. The resulting fission products can be sent to a repository and the transuranics left over can be mixed with transmutation fuel from the UREX separation and re-introduced into the first reactor for further transmutation.
Transmutation fuel that has been removed from the first reactor after a three year residence time is then introduced into a second reactor for further transmutation. The second reactor includes a sealable, cylindrical housing having a window to allow a beam of protons to pass through the window and into the housing. A spallation target is positioned inside the housing and along the proton beam path. Fast neutrons are thereby released when the beam of protons enters the housing and strikes the spallation target.
Graphite blocks containing the transmutation fuel are positioned inside the housing at a distance from the spallation target. A relatively low ratio of graphite mass to transmutation fuel mass is used in the second reactor to allow epithermal neutrons to reach the transmutation fuel. However, enough graphite is used to achieve the desired moderation for transmutation, with the attendant effect that fast neutron damage to reactor structures and equipment is limited. After a residence time in the second reactor of approximately four years, the reacted transmutation fuel is removed from the second reactor and sent directly to a repository. The spherical particles of transmutation fuel are coated with an impervious, ceramic material which provides for long-term containment of the reacted transmutation fuel in the repository.